Nuclear magnetic resonance (NMR) spectrometers typically include a superconducting magnet for generating a static magnetic field B0, and an NMR probe including one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample to the applied magnetic fields. Each RF coil and associated circuitry can resonate at the Larmor frequency of a nucleus of interest present in the sample. The RF coils are typically provided as part of an NMR probe, and are used to analyze samples situated in sample tubes or flow cells.
An NMR frequency of interest is determined by the nucleus of interest and the strength of the applied static magnetic field B0. In order to maximize the sensitivity of NMR measurements, the resonant frequency of the excitation/detection circuitry is set to be equal to the frequency of interest. The resonant frequency of the excitation/detection circuitry varies asν=1/2π√{square root over (LV)}  [1]where L and C are the effective inductance and capacitance, respectively, of the excitation/detection circuitry. Additionally, in order to maximize the transfer of RF energy into the RF coils, the impedance of each coil is matched to the impedance of the transmission line with a network of components electrically connected to the RF coil. If the coil is not impedance-matched, a sub-optimal fraction of the RF energy sent to the coil actually enters the coil. The rest of the energy is reflected, and does not contribute to the NMR measurement. Variable and fixed capacitors as well as inductors may be used to set the NMR circuit resonant frequency to desired values and to ensure optimal impedance matching.
Some NMR systems employ a cryogenically-cooled NMR probe. A cryogenic fluid such as liquid nitrogen or liquid/gaseous helium conductively cools NMR probe components such the NMR RF coils, circuits and preamplifiers. The sample of interest may be held at room temperature or at a different temperature than the cryogenically-cooled circuit components. Low-temperature probes commonly allow a reduction in the coil electrical resistance, and achieve relatively high Q-values. At the same time, as the coil thermal noise is reduced, other noise contributions may become increasingly significant.